Frequency dependent damping network



Patented Dec. 5, 1939 UNITED STATES PATEllT OFFiCE FREQUENCY DEPENDENT DAMPING NETWORK Holger'Lueder, Berlinsignor to Siemens &

Friedenau, Germany, as- Halske Aktiengesellschaft,

Siemensstadt, near Berlin, Germany, a. corporation of Germany I 10 Claims.

It has already been proposed to produce fre quency-dependent damping networks wherein coils or condensers are inserted in the .series and/or parallel train of networks. However, these arrangements have the disadvantage that the inductive resistance of coils, and the conductance of condensers increase in direct proportion with the frequency of the alternating currents passing therethrough. In many cases, in order to reduce distortion, it would be of advantage to employ electric networks with resistance elements whose dependence on frequency is non-linear. In this way with networks of very simple construction results could be obtained such as can today only be achieved with coils, condensers and frequency independent resistances most of which are required to be of unwieldy size and are, therefore, of costly construction.

My invention will be explained in further detail, reference being made to the accompanying drawing, in which Figures 1, 2, 3, 4 and 12 show, respectively, different filter characteristics by which the theory underlying my invention is explained;

Figs. 5 to 10 inclusive show, respectively, different embodiments of a filter network in accordance with my invention; and,

Fig. 11 illustrates a further modification of my invention in which a temperature-dependent resistance is preferably employed.

In accordance with the invention it is proposed to provide a simple filter system in'which distortion is minimized. The invention is par- ,ticularly adapted to the reduction of distortion in telegraph lines. It is based upon the teaching that, as shown in Fig. 1, a resistance with a negative temperature coemcient whose current-voltage characteristic has a horizontal course (curve 1) ofiers a negligible impedance to alternating currents passed across the resistor at a preliminary load according to the current 13, if the frequency of the alternating current is very low, but in case that alternating currents of higher frequency operate in the working point, an alternating current resistance is produced which corresponds to the lines 02 in Fig. 1. Theoretically this value appears only at infinitely high frequencies, but in practice this value will be reached in ordinary hot conductors at frequencies of the order of several kilocycles. In

' the region of transition a dynamic characteristic of elliptic shape is obtained (curve 3 of Fig. 1). When using other preliminaryload currents, for

1 instance, II or I alternating current resistthe root of the frequency of the telegraph cur-- rents, except as already pointed out, that it ap-- proaches asymptotically a finite value.

In the Figs. 3 and 4: corresponding conditions are represented for temperature dependent resistence with highly positive temperature coefficient. If, in this case, in accordance with Fig. 3, a static characteristic is used, namely, one in which the voltage characteristic takes a horizontal course, then as shown in Fig. 4, the con ductance G of such resistance, in the same manner as the resistance of the hot conductor according to Fig. 1 and Fig. 2, takes the same course as the resistance of a hot conductor according to Fig. 2. On the basis of the teaching just described it is proposed, in accordance with the invention, to produce frequency dependent damping networks applicable with advantage for various technical purposes, in that temperature dependent resistances are connected in series and/or parallel branches with eifective resistances, and that the working point of the temperature dependent resistances is so chosen that a desired dependence on frequency of the damping network is obtained. Especially suited for this purpose are temperature dependent resistances whose characteristic traced at direct current (static characteristic) reveals a constant voltage or a constant current within a wide range.

In order to illustrate the possibilities for employing temperature-dependent resistances in a manner required for carrying out my invention it should be mentioned that a number of materials have been experimented with and among them the following have been found useful: uranium dioxide, chromium oxide, molybdenum dioxide, titanium oxide and zirconium oxide. The particular materials just mentioned fall within the group of non-linear resistances having a negative temperature coefiicient. They are sometimes referred to as hot conductors as contrasted with so-called cold conductors having a positive temperature coeificient. It should be understood also that the resistance temperature characteristic of the material that is used is to some extent determined by the dimensions and the shape of the element which is formed into a resistor.

A number of difierent embodiments of my invention are shown in Figs. 5 to 11 inclusive. These figures show diiferent arrangements of resistance units connected in the series and parallel branches respectively of a filter system. Each of the filters shown has-input terminals on the left and output terminals on the right of the diagram. It will be understood that any suitable load is connected to the output terminals. In. each: of

these figures the resistance unit designated- W is one which has a substantially zero temperature coeificient. The resistance H, on the other hand,

has a negative temperature coefficient and is sometimes termed a hot conductor. The resistances designated K are cold conductors or resistance units having a positive temperature coefficient.

Due to the disposition of these resistors in the arrangements of Figs. 5, 6 and 7, all of the filters therein shown are high pass filters. Thus, in Fig. 5 the resistor W having a substantially zero temperature coefiicient is placed in the series arm, while the resistance I-I having a negative coefiicient is placed in a parallel arm of the filter. In Fig. 6 a cold conductor having a positive temperature coefiicient is in one of the series arms, while a resistance W having a substantially zero temperature coefficient is placed in a parallel'arm. In Fig. 7 the arrangement is similar to that of Fig. 6 except that a hot conductor having a negative temperature coeificient is substituted for the resistor W of Fig. 6 in the parallel arm.

Low pass filters are represented in each of the embodiments shown in Figs. 8, 9 and 10. Her-e the resistor H of Figs. 8 and 10 is placed in a series arm and a resistor W as in Fig. 8, or a resistor K as in Figs. 9 and 10, is placed in a parallel arm. It will be remembered that the resistor W has a substantially zero temperature coefiicient, while the resistor K is a cold conductor having a positive temperature coefficient. In Fig. 9 the resistor W. is placed in a series arm of the filter.

Fig. 11 shows another high pass filter which comprises a resistor W having a zero temperaturev coefiicient and a resistor H having a negative temperature coeihcient, both of these resistors.

being placed in a parallel arm and in series with one another. The resistor H, however, is preferably shunted by a circuit which includes a choke D, a direct current source B and an adjustable potentiometer R. The utility of this shunt circuit around the resistor H will nowbe explained. I

'In order to be able to set the working point of the temperature dependent resistances in the desired manner, a variable bias potentialor a variable preliminary current is applied in the'known manner to the temperature dependent resistances.

This bias potential is supplied by the battery B. The choke is preferably inserted for thepurpose of avoiding an alternating current short circuit through the battery. The value of the bias potential is controlled by the adjustment of the potentiometer R. rangement shown in Fig. 11 the circuit or device whose internal resistance is to bev compensated is designated W. This unit may be either a generator or a telegraph line or any other unit which is subject to variations of its impedance due to frequency variations. In such cases. the parallel arm of the filter is effective to reduce distortion- In the same manner the preliminarycurrent' or According to. the ar-' damping increase in telegraph lines, especially in telegraph cables, has a characteristic that is approximately inversely proportional to the damping increase of a high pass filter constructed by the elements proposed in accordance with the invention.

When negative resistances are inserted in telegraph'lines. or cables. for the purpose of damping reduction, then it is with particular advantage that the; temperature dependent resistances of my invention may be employed. This is true because there-are certain indeterminate components of the resistors having a negative temperature coefficient which can be utilized in a compensating manner for the reduction of distortion. The high pass filter shown in Fig. 11 illustrates a preferred system to be so employed. The ohmic resistor W prevents a short circuit of the low frequencies which are fed to the line or cable.

Such av short circuit is otherwise substantially offered at low frequencies by thehot conductor H having av negative temperature coefficient.

By the choice of the material, the dimensioning of the hot conductors, as well as by the choice of the most favorable Working point andv by the proper dimensioning of the, series resistances W, it is always possible to easily obtain a distortion reducing network which is matched to a wide degree with the damping curve (b) of the cable.

Fig. 12 shows, by way of example, the damping curveof a Pupinized telegraph cable P and the conductance Gof a high pass filter constructed.

in accordance with the invention. It. is found that in the entire range required for the transmission of audible speech currents both curves have prac tically the same course.

I claim:

1. In circuit with a load. having a non-linear frequency-response characteristic, a, frequency dependent damping network. comprising a plurality of temperature-dependent. non-reactive re sistors. in seriesjand parallel. branches of said network, and means for so adjusting the resistance-temperature characteristic offatieast one of said resistors that. said network possesses a desired damping characteristic dependent upon frequency. variations of the energy impressed thereon.

2.'A network in accordance with claim 1 and further characterized in that the stationary current-voltage characteristic of the temperature dependent resistor follows a substantially horizontal course within a. predetermined range of frequencies.

3. A frequency dependent damping network in accordance with claim 1, and having at least.

one of I the resistors of said network such that it possesses a negative-temperature coefficient.

4. A high-pass'filter system having a plurality ofseriesiand. parallel disposed impedances,.one

of said-impedancesi being a temperature-dependent resistance. having a positive temperature coefiicien tand disposed in the series train, another of said impedances having a resistive characteristic substantially independent of temperatureand disposedin-the parallel train.- of said net-- work, and means for so adjusting the working point of. the temperature-dependent resistance that a desired high-pass characteristic is obtained' in said" filter system".

5. A frequency dependent damping network having series and parallel branches, characterized in that non-reactive temperature dependent resistances are connected together with effective resistances in at least one of said branches, and that the working point of the temperature dependent resistances is so chosen that a desired dependence on frequency of the damping network is attained.

6. A network according to claim 1 characterized in that the stationary current-voltage characteristic of the temperature dependent resistance is substantially linear within the operational range of said characteristic.

7. A damping network comprising a series resistor, a parallel resistor, input terminals connected to a variable frequency alternating current source, and output terminals connected to a load having a variable frequency response char- 20 acteristic, at least one of said resistors having a relatively high temperature coefficient and a frequency-dependent damping characteristic and being relatively non-reactive.

8. A damping network according to claim 7 wherein one of said resistors possesses a useful negative temperature coefficient and the other of said resistors possesses a substantially negligible temperature coeflicient.

9. A damping network according to claim 7 wherein the parallel resistor possesses a higher negative temperature coeflicient than that of the series resistor, whereby a high-pass filter characteristic is obtained in said damping network.

10. A damping network according to claim 7 wherein the series resistor possesses a higher negative temperature coefiicient than that of the parallel resistor, whereby a low pass filter characteristic is obtained in said damping network.

HOLGER LUEDER. 

